Try to book your excursions online before you sail. I waited until we were on the ship and the ones I wanted were sold out. We did do the Circle of Fire helicopter tour and it was very good. We're sorry we didn't do Waimea Canyon on Kauai. I did not notice any tour operators at the dock on that island.
I did do Captain Steve's Whale watching in Maui on my own and not only did we see a lot of whales, it was cheaper than the ship. You could see my review on this on this site.
If you want to do independent tours in Honolulu, get off the ship early and head to the little hut next to the pier by the shopping mall.